Field of the Invention
The present invention relates to a liquid crystal display panel having a driving circuit therein and a method of forming a driving IC (integrated circuit) chip on a glass substrate of a liquid crystal panel during formation of an IC circuit module, and more particularly to a bump structure formed on an electrode of a driving IC chip and a method of forming the same wherein the driving IC chip is formed on the pad of a glass substrate using an anisotropic conductive film (ACF) to improve electrical contact between the bump and the pad, prevent shorts between the bumps, increase manufacturing yield, and increase operational efficiency of the conductive lines in the module.
Among the display devices, a Cathode Ray Tube (CRT) is most commonly used as a display device for a TV or a computer monitor because of its easy color formation and high response time. However, the CRT requires a predetermined distance between an electron gun and a screen, which results in a relatively thick CRT. This increases the weight of the device, decreases its portability, and increases power consumption. In order to overcome the above limitations associated with the use of a CRT, use of a liquid crystal display has been proposed.
There are several distinctions between a CRT and a liquid crystal display. A conventional liquid crystal display displays a darker picture and has a somewhat lower response time when compared to the CRT. Nonetheless, the conventional liquid crystal display does not require an electron gun and each pixel can be selectively operated based on the signal from a common bus line. The liquid crystal display is an ultra-thin display device, suitable for displaying on a wall or for use in all sizes of televisions. Moreover, the conventional liquid crystal display is relatively light-weight and since its power consumption is low compared to the CRT, it can be efficiently operated using merely a battery. Therefore, the liquid crystal display is well suited for use in a notebook computer or in a system requiring a portable display device.
FIG. 1 shows the structure of a typical liquid crystal display. In a typical liquid crystal display, a plurality of scanning lines (gate lines) 104 and a plurality of data lines 106 crossing the gate lines 104 are formed in a matrix on a first substrate 100. At each crossover region, a pixel electrode 108 and a thin film transistor 109 (sometimes hereinafter referred to as "TFT") are formed. Facing the first substrate 100, a second substrate 102 (having a common electrode 110 and color filter 112) is formed. Between the first and second substrates 100 and 102, a liquid crystal 114 is injected to form the liquid crystal panel 130 (See FIG. 3). The liquid crystal along with the pixel electrodes 108 and the common electrode 108 function as pixels for the liquid crystal display panel. On the outer surfaces of both the first and second substrate 100 and 102, polarizing layers 116 are disposed to control selective transmission of light.
FIG. 2 shows a cross-sectional view of a typical thin film transistor used in the liquid crystal panel of FIG. 1. As shown in FIG. 2, the TFT includes a gate electrode 120, a source electrode 121, a drain electrode 122, and a semiconductor channel 123. The gate electrode 120 is formed of a metal such as chromium. The source and drain electrodes 121 and 122 are formed of a metal or a transparent conductive material such as indiumtinoxide (ITO) The gate electrode 120 is connected to a corresponding gate line 104, the source electrode 121 is connected to a corresponding date line 106, and the drain electrode 122 is connected to a corresponding pixel electrode 108.
When a scanning voltage is applied to the gate electrode 120 through the gate line 104, it allows a voltage representing data from the data line 106 to transmit from the source electrode 121 to the drain electrode 122 through the semiconductor channel layer 123. The signal voltage transmitted to the drain electrode 122 causes an electric potential difference between the pixel electrode 108 connected to the drain electrode, and the common electrode 110 formed on the second substrate 102. The electric potential difference causes a change in the orientation of the liquid crystal molecules disposed between the pixel electrode 108 and the common electrode 110. As the orientation of the liquid crystal molecules changes, the light transmittance through the liquid crystal changes. Accordingly, the TFTs of the liquid crystal display panel collectively function as a switching element for selectively driving the pixels of the liquid crystal display.
As shown in FIG. 3, the conventional liquid crystal display includes a liquid crystal display panel 130 for displaying images and driving ICs 131 and 13 for generating an image signal. The typical liquid crystal display includes a first pad 133 connected to the gate lines 104 and a second pad 134 connected to the data lines 106. The first pad 133 is connected to a first IC 137 which generates a scanning signal, whereas the second pad 134 is connected to a second IC 138, which generates a data signal. The scanning signal generated from the first IC 137 is transmitted to the gate lines 104 through the first pad 133. The data signal generated from the second IC 138 is transmitted to the data lines 106 through the second pad 134. There are many methods for connecting the first pad 133, or the second pad 134, with the first IC 137 or the second IC 138. However, in order to connect the IC with the pad, it is necessary to form a bump at an electrode of the IC prior to the connection between them.
FIGS. 4A-4C are cross-sectional views for illustrating a typical method of forming a bump for a liquid crystal display. As shown in FIG. 4A, a protective layer (passivation layer) 145 is formed over a surface of the IC 140, except in an area where the electrode 141 is formed. On the protective layer 145 and over the electrode 141 of the IC, a metal 142 is deposited. A photoresist 143 is then applied over the metal 142, which defines a region in which a bump is to be formed. The metal 142 serves to ensure a uniform deposition of gold in subsequently forming the bump by electrodeposition.
As shown in FIG. 4B, a gold (Au) is deposited by electrodeposition on the metal 142 exposed through the photoresist 140 formed on the IC 140. Since the photoresist 133 is made of an insulating material, gold is not deposited on the photoresist 140 during electrodeposition. Therefore, gold is deposited on a portion of the metal 142 corresponding to the electrode 141 of the IC so as to form a bump 144. Here, it is preferred that the height of the bump 144 is greater than the height of the photoresist 143. In which case, the bump will slightly overlap the photoresist 143. The reason for forming the bump 144 to slightly overlap the photoresist 143 is to enhance contact with other electrodes. As described above, the bump is preferably made of gold. However, other materials having similar properties may also be used such as copper Cu or nickel Ni.
As shown in FIG. 4C, once the bump 144 is formed, the photoresist 143 is completely removed by chemical stripping. Those portions of the metal 142 outside the bump 144 are then removed, so that only the bump 144 (made of gold supported on a portion of the metal 142) remains on the electrode 141 of the IC.
Generally, a TAB (tape automated bonding) process is used to connect the bump 144 on the IC 140 with the pads. The TAB process involves an ILB (inner lead bonding) step, and an OLB (outer lead bonding) step. The ILB step includes attaching a carrier film lead to the electrode of the IC through the bump. After the ILE step is completed, the OLB step attaches the lead of the TAB package connected to the pad of the liquid crystal display panel.
FIGS. 6A, 6B and 6C are cross-sectional views for explaining a conventional TAB process.
As shown in FIG. 6A, a bump 144 is formed on the electrode 141 of the IC to be connected with a pad of the liquid crystal panel. The bump is formed by using the method previously described referring to FIGS. 4A-4C.
As shown in FIG. 6B, an edge of a polyimide film including a plurality of metal lines is correspondingly aligned above each of the bumps. The metal lines 151 are attached to the polyimide film 150 by an adhesive (not shown). The metal lines 151, as attached to the polyimide film 150, function to transmit a signal generated from the IC.
As shown in FIG. 6C, the polyimide film 150 is attached to the bump by a bonding process to cause an electrical short. During this process, in order to attach the polyimide film 150 with the bump 144, a heat process method is utilized to make an eutectic alloy of the portion between the bump and the lead for bonding the film 150 to the electrode of the IC. Further, to achieve protection from external impact and to resist moisture, a protective resin 155 is formed to encapsulate the bonding areas.
FIG. 5 shows the connection between the LCD panel and a bumps. Once one end of the polyimide film 150 is bonded to a corresponding bump 144, the other end of the polyimide film 150 is bonded to the pad of liquid crystal panel 170 using an anisotropic conductive film (ACF) (not shown). During this process, it is necessary to align the metal line so that the metal line shorten with the electrode of the IC is in contact with the pad through conductive balls in the ACF. This TAB process allows the IC to be disposed externally to the liquid crystal panel and a short between the electrode of IC and the electrode of the liquid crystal panel through the polyimide film with the metal line bonded thereto.
In contrast to the TAB process, another method for attaching IC's to an LCD panel is the COG (chip on glass) process which adheres an IC directly on a glass substrate of a liquid crystal panel. The COG method does not utilize the polyimide film used in the TAB process, but attaches the bumps to the pads using ACF to bond the IC on the glass substrate. The COG method is both simpler and less expensive than the TAB method since the use of the polyimide film is eliminated. It also decreases the space occupied the liquid crystal display making the finished device more suitable for smaller devices.
FIGS. 7A and 7B are cross-sectional views for explaining a method of bonding an IC using the COG method.
As shown in FIG. 7A, an ACF is placed on the glass substrate 180 and pads 181 of the liquid crystal display panel. The bumps 144 supported by the IC 140 are aligned with the pads 181 of the glass substrate 180. The ACF 153 occupies the area between the IC 140 and the substrate 180. The ACF 153 contains a plurality of conductive balls 154 (also referred to as conductive particles) which are disbursed throughout the film 153. The bump 144 is formed as shown in FIG. 4.
With reference to FIG. 7B, the IC is bonded to the glass substrate using heat process. Conductive balls (or conductive particles) 154 dispersed in the ACF 153 are suppressed between each of the bumps 144 and the pads 181 such that the conductive particles collectively establish a short between the pads 181 and the electrodes of the IC. As a final step, heat is applied to the IC for hardening the softened ACF to form a securely bonded IC on the pad of the glass substrate.
The TAB method is useful for forming a reliable connection between the ICs and the liquid crystal display panel. However, the TAB method has the disadvantage of making the process of assembling the LCD more cumbersome by requiring a first step for applying the polyimide film and a second step for applying the ACF resin. Therefore, to utilize the TAB method, the number of steps required for manufacturing a liquid crystal display is increased. Further, the polyimide film adds significantly to the cost of assembling the LCD. Moreover, because the polyimide film is required to connect the IC, the size of the liquid crystal display is considerably increased.
The COG method overcomes many of the problems associated with the TAB method by directly attaching the IC to the pad of the glass substrate. However, the COG bonding method has the following problems. As illustrate in the portion of FIG. 7B within the dotted line circle "A", the resin begins to flow when heat is applied to the IC for bonding the ACF above a glass transition temperature Tg. This flow phenomenon is depicted by the arrow 183. As the ACF flows, the conductive balls (conductive particles) suspended in the ACF also flow to migrate to the open spaces between the bumps. The migration of the conductive balls causes a problem near the peripheral edge portions of the bump head so that the electrical contact between the bump and the pad is poor due to less number of conductive balls remaining thereof. Furthermore, when the migrated conductive balls concentrate in the open spaces between adjacent bumps, an electrical short between the bumps occurs. Consequently, the contact resistance can be increased and leakage of signal can occur.